An adult human contains about 5 liters of blood, of which red blood cells account for about 45% of the volume, white cells about 1%, and the balance being liquid blood plasma. Blood also contains large numbers of platelets. In view of the substantial therapeutic and monetary value of blood components such as red blood cells, platelets, and plasma, a variety of techniques have been developed to separate blood into its component fractions while ensuring maximum purity and recovery of each of the components.
Typically, donated blood is collected in a blood collection bag and separated by centrifugation into PRC and platelet-rich plasma (PRP) fractions, the latter of which is in current practice separated by a second centrifugation to provide plasma and PC. With respect to the second centrifugation step, the platelet concentrate is typically obtained from the PRP by "hard-spin" centrifugation (rotating at about 5000 G). This hard-spin compacts the platelets into a pellet or concentrate at the bottom of the test tube, flask, or bag. The plasma component is then removed or expressed to a separate bag or container, leaving the platelet component and some plasma behind. This platelet composition, which tends to form a dense aggregate, is subsequently dispersed to make PC. The dispersion step is usually carried out by gentle mixing, for example, by placing the bag on a moving table which rotates with a precessing tilted motion.
Platelet concentrate can also be prepared using apheresis of autologous blood. With this method, whole blood is removed from a single donor, and centrifuged into its component parts. The desired component is then harvested and the remainder of the blood is returned to the donor. This procedure allows collection of multiple units from one donor. Typically, a 2 to 3 hour apheresis procedure will produce a platelet product containing 3.times.10.sup.11 platelets, equivalent to about six to ten units of random donor platelets, i.e., a typical transfusion unit. The common practice with respect to platelet concentrate is to transfuse a pool of six to ten units of platelets per administration, containing a total of about 300 to 700 ml of platelet concentrate.
Blood bank personnel have responded to the increased need for blood components by attempting to increase packed red cell (PRC) and platelet concentrate (PC) yields in a variety of ways. For example, in separating the PRP from PRC, blood bank personnel have attempted to ensure that the entire PRP fraction is recovered, but this may be counter-productive, since the PRP, and the PC subsequently extracted from it, are frequently contaminated by red cells, giving a pink or red color to the normally light yellow PC. The presence of red cells in PC is so highly undesirable that pink or red PC is frequently discarded, or subjected to recentrifugation, both of which increase operating costs and are labor intensive.
Additionally, freshly donated blood contains platelets varying in age from newly-formed to 9 days or more in age. Newly-formed platelets are larger, and are generally believed to be more active. Because the younger platelets are larger, they tend to sediment faster during the first centrifugation step, and consequently are present in larger numbers in the PRP nearest to the red cell interface. Thus, although it is desirable to reclaim a larger proportion of the younger, more active platelets, attempting to obtain a greater quantity poses a risk of contamination with red cells.
Typical techniques for processing platelets or platelet concentrate may reduce the yield and/or adversely affect the platelets. For example, as noted earlier, during the separation of PRP from PRC, it is difficult to efficiently obtain the maximum yield of platelets while preventing red cells from entering the plasma. Additionally, hard spin centrifugation and dispersion is labor intensive and it is believed that the forces applied during centrifugation may damage the platelets. For example, the hard-spin is potentially damaging to the platelets as it induces partial activation agglomeration of the platelets and may cause physiological damage. Such agglomeration requires several hours to resuspend the platelets in solution before they can be used for transfusion into a patient. Additionally, the several hour dispersion step is an undesirable delay, and is believed by many researchers to partially aggregate the platelet concentrate.
Furthermore, the hard-spin typically produces "distressed" platelets which partially disintegrate upon resuspension. Unfortunately, while mixing does prevent agglomeration, it encourages gas exchange by diffusion of oxygen through the walls of the bag (thereby controlling pH), and bathes the product in needed nutrients, this requires time, resulting in an increase in the number and size of microaggregates. Further, over time, gel-like bodies may be formed, which may comprise fibrinogen, degenerated protein, and degenerated nucleic acids, which may interfere with the separation of the platelets from the donated blood. Thus, some platelets are lost due to the process conditions.
Additionally, since platelets are notorious for being "sticky", an expression reflecting the tendency of platelets suspended in blood plasma to adhere to any non-physiological surface to which they are exposed, the recovery of platelets may be adversely affected during the preparation of platelet concentrate, regardless of the method of preparation. Furthermore, under many circumstances, platelets also adhere strongly to each other. Accordingly, in recovering platelets, it is desirable to restrict platelet loss to about 15% or less of the original platelet concentration.
Moreover, while leukocyte depletion of blood components for transfusion may decrease risk to the patient, when leukocytes are removed from, for example, platelet-rich plasma, which typically results in the production of a leukocyte-free platelet concentrate, the platelet component of the filtrate usually passes through a filter or separation device. In these systems, platelets may adhere to the surfaces of components of the separation device; such adhesion tends to cause substantial, and sometimes complete, removal of platelets from the filtrate. Furthermore, platelet concentrate present within the separation device at the completion of the separation process will be lost.